CONTEXT: Cash transfers conditional on certain behaviors, intended to provide access to social services, have been introduced in several developing countries. The effectiveness of these strategies in different contexts has not previously been the subject of a systematic review. OBJECTIVE: To assess the effectiveness of conditional monetary transfers in improving access to and use of health services, as well as improving health outcomes, in low- and middle-income countries. DATA SOURCES: Relevant publications were identified via electronic medical and social science databases from inception to April 2006 (PubMED, EMBASE, POPLINE, CAB Direct, Healthcare Management Information Consortium, WHOLIS (World Health Organization Library Database), African Healthline, International Bibliography of the Social Sciences (IBSS), Eldis, British Library for Development Studies (BLDS), ID21, Journal Storage (Jstor), Inter-Science, ScienceDirect, Internet Documents in Economics Access Service (Research Papers in Economics) (IDEAS[Repec]), Latin American and Caribbean Health Sciences Literature (LILACS), MEDCARIB, Virtual Library in Health (ADOLEC), Pan American Health Organization (PAHO), FRANCIS, The Cochrane Central Register of Controlled Trials, the Database of Abstracts of Reviews of Effectiveness, and the Effective Practice and Organization of Care Group (EPOC) Register. Reference lists of relevant papers and "gray" literature resources were also searched. STUDY SELECTION: To be included, a paper had to meet study design criteria (randomized controlled trial, interrupted time series analysis, and controlled before and after study) and include a measure of at least 1 of the following outcomes: health care utilization, health expenditure, or health outcomes. Twenty-eight papers were retrieved for assessment and 10 were included in this review. DATA EXTRACTION: Methodological details and outcomes were extracted by 2 reviewers who independently assessed the quality of the papers. RESULTS: Overall, the evidence suggests that conditional cash transfer programs are effective in increasing the use of preventive services and sometimes improving health status. CONCLUSIONS: Further research is needed to clarify the cost effectiveness of conditional cash transfer programs and better understand which components play a critical role. The potential success and desirability of such programs in low-income settings, with more limited health system capacity, also deserves more investigation.